Re: forward declaration of class from another namespace

Nick Hounsome <>
Sun, 25 Oct 2009 13:26:51 CST
On 23 Oct, 21:34, Sergey Lukoshkin <> wrote:

Hello, dear community!

I've faced such a problem:

1. I got header "headerA.h" with the following code:

#include "headerB.h"

namespace A
       class A

                    B* pB = new B( this );



2. And I got header "headerB.h" with the following code:
namespace B

     class B
             B( /* here I need to use pointer to object of type A::A
*/ ){....}


what is the correct way to make forward declaration of class A::A in
module B?
Thanks in advance!!

It is invariably bad design to have 2 mutually dependent classes
unless they are part of the same logical component (e.g. a list and a
link/ a tree and a node) in which case they should be in the same
header as they are not designed to be used independently.

It is bad design because neither class can be tested or linked
independently so the separation into two headers has no logical
function other than to reduce the size of the files.

One way to break such mutual dependencies is to derive the concrete
classes from interface classes at a lower level either one way or both

// headerAInterface.h
class AInterface { /* virtual methods */ };

// headerA.h
class A: public AInterface { /* implement methods */ }

// headerB.h
#include "headerAInterface.h"
// pointers to AInterface only

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